Nearly every person living long enough develops some degree of osteoarthritis, sometimes referred to as degenerative or wear-and-tear arthritis in both the knees and hips. Osteoarthritis becomes disabling when the articular cartilage covering the joint degenerates, resulting in areas of the joint where bone rubs against bone. Knee osteoarthritis in particular, which most typically occurs on the medial aspect of the knee, accounts for more disability with respect to mobility than any other disease in the elderly. That the medial aspect of the knee is particularly susceptible is due to the fact that during walking (and other weight bearing activities), an external varus knee torque throughout the stance period imparts a continuous compressive force across the medial aspect of the knee. While the medial aspect of the knee is most susceptible to arthritis, the varus torque makes the entire knee susceptible by necessitating counterbalancing muscle activity that imparts substantial muscle compressive forces throughout the knee.
Other than avoiding acute joint trauma and maintaining normal body weight, there are no proven measures to reduce the inevitable wear and tear during weight bearing activities of the knee such that the gradual development of knee osteoarthritis can be prevented or mitigated. Footwear or shoes often include material in the soles that absorb shock. However, because shock absorbing material does not actually reduce joint torques or forces during walking or other weight bearing activities, the propensity to knee and hip osteoarthritis is unchanged. Although shoe designs and arch supports often support the medial side of the foot, including the natural arch of the foot, they affect only the anatomy of the foot and do not reduce (and in fact can increase) the knee varus and hip adductor torques.
A laterally wedged heel or sole can reduce knee varus torque. While a wedge-like sole could be acceptable as a treatment modality for a person who already has knee osteoarthritis, the shoe insert alone is unlikely to be an acceptable preventative device that could be useful to a healthy person in preventing knee osteoarthritis. In use, such a shoe insert or design will tend to pronate the foot and collapse the natural arch. Such a tendency toward collapse of the foot arch with repetitive steps is likely to be uncomfortable for healthy people and athletes, and probably does not warrant the risk of foot injuries and deformities. Moreover, the end result of a collapsed foot arch would be an increase rather than a decrease in the knee varus and also hip adductor torques.
Thus, there is a need for a footwear design that both comfortably and adequately supports the foot and prevents pronation, yet also reduces the external knee varus torque and potentially also reduces the hip adductor torque and ankle inversion torque. By reducing the knee varus and hip adductor torque, less counterbalancing muscle activity would be required, resulting in reduction of the forces throughout the knee (not just the medial side) and hip. Such a design would be particularly useful for prevention of both knee and hip osteoarthritis and could help prevent common musculoskeletal injuries including hip pointers and ankle sprain injury. By reducing the joint torques in the coronal plane (knee varus, hip adductor and ankle inversion), the design would simultaneously reduce the need for counterbalancing muscle activity (and strain) in the coronal plane to maintain posture, thus improving muscle efficiency and athletic performance during weight bearing activities. There is currently no effective shoe design that reduces the need for muscle activity related to the coronal plane.